A nonagglutinating lectin, P-4, specifically recognizes surface glycoconjugates on human monocytes. The binding of this lectin can serve as a useful marker for human monocytes. The covalent coupling of Pa-4 to a solid support has provided an easy, reversible procedure for the removal of human monocytes from peripheral mononuclear cells. Studies using neoglycoproteins (synthetic glycoproteins with selected carbohydrate moieties) have shown that subsets of human B and T cells can be defined by their surface carbohydrate receptors (specific endogenous lectins). Further, three neoglycoprotein probes have been synthesized which are bound by human null cells. Human monocytes, and a subset of suppressor T cells, bind a complex carbohydrate isolated from pokeweed and can be specifically and completely removed from PBL by reversible agglutination with that carbohydrate. When placed in culture, these T cells produce a potent, soluble L-rhamnosespecific suppressor factor which shuts off immunoglobulin by B cells.